The direct coating of aluminum or aluminum alloys by means of organic coating systems is virtually impossible on account of the inadequate bonding of the organic coating systems to the aluminum. Therefore, to improve the bonding between the starting material and organic coating systems, it is known to subject aluminum or aluminum alloys to what is known as a boehmite treatment, in which hot water or hot steam, if appropriate together with ammonia or amines, is brought into contact with the workpiece, so as to form or thicken an aluminum oxide or boehmite layer. This then allows an organic coating to be applied. EP 1 142 663 A1 describes boehmite processes, in which deionized water at temperatures of approximately 100° C. or steam at temperatures of 150° C. is used to surface-modify aluminum parts. U.S. Pat. No. 3,945,899 has disclosed a boehmite treatment of aluminum parts with water at a temperature of from 65 to 100° C. or steam at temperatures of 100 to 180° C., with the addition of amines and ammonia further boosting the aluminum oxide layer.
It is also known from this document to subject aluminum or its alloys to a chemical surface treatment using aqueous solutions of chromates or phosphates, in order in this way firstly to increase the bonding and secondly to reduce the susceptibility to corrosion. This conversion treatment, as it is known, is also known from Stolzenfels (Industrie-Lackierbetrieb, [Industrial coating operation] No. 3, pp. 93-98, Curt R. Vincentz Verlag), which describes chromating treatments of aluminum workpieces at temperatures of from 20 to 50° C. Riese-Meyer et al. (Aluminium 1991, No. 12, pages 1215-1221) describes chemical conversion treatments by means of layer-forming phosphating steps and chromating steps, allowing the coating bonding and corrosion resistance of aluminum workpieces to be improved. According to this document too, the chromating is carried out at a temperature of from 20 to 30° C. or 30 to 40° C.